Recovery of vanadium and nickel from oil fly ash

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Abstract

Oil fly ash (OFA) is a by-product originated from burning of crude and residual oil for energy. OFA is defined as hazardous waste in many countries because it is acidic and contains harmful heavy metals, which raises the risk of its contamination to the surrounding environment. However, OFA contains up to 90% carbon and numerous studies have been conducted on developing adsorbents derived from OFA. The group in the Faculty of Engineering and Applied Science at Memorial University has been focusing on the research that utilizes modified OFA in water and wastewater treatment for the last seven years. However, the cleaning of the OFA prior to use has not been systemically investigated yet. In addition, studies showed that OFA can be a potential source for vanadium and nickel and it is possible to recover these metals from the OFA cleaning process, so that the economic value of the OFA can be further maximized.
The selection of OFA cleaning and metal (mainly vanadium and nickel) recovery processes is heavily dependent on the properties of the OFA such as ash content, metal composition & oxidization state, sulfur content, pH, and particle size. Although various methods with high recovery rates had been proposed in literatures, they may not obtain the same result with different OFAs. Therefore, the purpose of this study is to develop a practical integrated cleaning and metal (vanadium and nickel) recovery process specifically for the OFA that the group has been dealing with. In this work, the leachability of five major metals (vanadium, nickel, iron, magnesium, and aluminum) by sulfuric acid, sodium hydroxide and ammonia at different concentrations were tested. After identifying the preferred leaching conditions, three metal recovery methods (i.e., chemical precipitation, ion exchange resin and solvent extraction) were evaluated. Based on the results, three complete metal leaching and recovery processes were proposed and their performances were assessed by mass balance calculation.
Briefly, the study showed that both strong acid and strong base can effectively extract vanadium from the OFA, however, base leaching may require additional leaching processes to recover nickel and carbon. Therefore, strong acid, sulfuric acid in this case with concentration ranging from 0.5N to 1N, solid/liquid ratio of 1:10 and mild heating, was selected for the sequential selective metal recovery tests. Among those three metal recovery processes, solvent extraction showed better extraction rate and selectivity. Between the selected solvents, Trioctylamine (TOA) is preferred over Di-(2-ethylhexyl) phosphoric acid (D2EHPA) for high iron OFA (SPP) as it only extracts vanadium (V), while D2EHPA co-extracts both vanadium and iron. The complete integrated process showed that 89.7% nickel and 59.2% of vanadium was recovered by acid (sulfuric acid) leaching and TOA solvent extraction. The study also found that the metal content in OFA has great impact on the recovery rate as higher metal concentration in the leachate leads to higher extraction and precipitation rates. Since the vanadium and nickel content in OFA varies greatly, a segregation process is recommended to treat high carbon OFA and high metal OFA separately.